Can You Tell Mary-Kate from Ashley?

As part of Slate’s twins issue this week, Barry Harbaugh writes about DNA tests that twins can take to determine whether they are fraternal or identical siblings. He found that most twins who believe they’re fraternal aren’t wrong about being dizygotic—it’s the identical twins who are more likely to be mistaken about their twinly identity. When multiples can describe themselves as being “like two peas in a pod,” he writes, they almost always end up having come from the same fertilized egg.

That got us thinking about the most famous twins in the United States: Mary-Kate and Ashley Olsen. Despite the fact that they look eerily similar—similar enough to have found fame playing a single character—the pair has always claimed, without citing scientific evidence, that they are, in fact, fraternal twins. (“We were just told by the doctors” that the girls were fraternal, says their dad, Dave Olsen, when reached today by phone. “We never thought about it much. They are good kids.”)